thanks for sharing info! So, the XML would go into a local module that we need to create, yes? (I found this by searching: goo.gl/UBFgS) And then can the public function be declared in the phtml that is running the cart code I'm contemplating?
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Chris KMay 8 '12 at 21:43

As righty answered by Gershon Herczeg, Jürgen Thelen and Xman Classical. You will need to write a observer of sales_quote_add_item event.
When any product is added to cart your observer will be triggered. If the product is Configurable then this event will be fired twice, You will have to something like that to get simple product only.

The issue regarding configurable product has been solved. You simply remove the

$quote_item->save();

and then for the product will not added to the cart twice. But one another very serious issue is remaining in this function. That is, with this function we can update the item price in cart but after added to cart, product price are not changed according the different currency. So this function can not be used for a store having multiple currency.

The question does not state whether this should be done by adding some logic to the code or not. So since you have answers for developer there is also something that is called cart price rules (in admin panel go to Promotions > Shopping Cart Price Rules) where you can create different rules for making sales and discounts (with or without coupons).